This invention relates to a method for providing steel with lowered hydrogen levels after ladle treatment. Specifically, the method involves covering molten steel in the ladle with a mixture which includes calcium aluminate and dolomitic limestone.
Various synthetic slags have been used to cover molten steel in the ladle to prevent heat loss and to increase sulfur removal during ladle treatment U.S. Pat. No. 4,795,491, Quigley, discloses a premelted synthetic slag for desulfurizing molten steel in the ladle. The premelted slags are obtained as the by-product from production of vanadium and ferrovanadium. They contain calcium aluminate and 9%-20% magnesium oxide. The reference cites as an advantage that magnesium oxide is less likely to absorb water of hydration than calcium oxide. Improved ladle processing is said to be expected from the avoidance of water in the ladle process. This reference does not suggest the use of dolomitic limestone in the synthetic slag composition. U.S. Pat. No. 4,417,924, Schwer, discloses a steelmaking additive for flux solubilization, desulfurization and/or the removal of inclusions. The additive may be added to the ladle in amounts of from 5 to 20 lbs. per ton of steel and contains 20% to 50% lime and 75% to 50% calcium aluminate. Dolomitic lime may be used containing about 58% CaO and 42% MgO. The lime may be in either calcined or hydrated form. The reference does not suggest the use of dolomitic limestone in the additive compositions. This reference also teaches that the additive should not contain any spar so as to avoid shortening ladle lining life.
Finally, an abstract of JP52099915, Nippon Steel Corporation, discloses a flux for reducing hydrogen pick-up for molten steel refining in the furnace or for ladle refining. The flux is made from quick lime reacted at high temperature with carbon dioxide gas to form 2% to 30% CaCO.sub.3. This component is then blended with a CaO--SiO.sub.2 system. For deoxidation of the steel, alumina is added in place of SiO.sub.2 in the ratio of CaO/Al.sub.2 O.sub.3 of 1 to 3. To lower the melting point of the flux, up to 20% of CaF.sub.2 may be added or a few percent of an oxide or carbonate of an alkali metal. This reference does not suggest a flux with calcium aluminate, nor one with magnesium oxide or dolomitic limestone. Another reference of interest is U.S. Pat. No. 4,373,949, Spruell, et al., disclosing the use of higher than normal amounts of dolomitic lime in a top blown oxygen steelmaking vessel.